Spent caustic is the hazardous and toxic waste product of gas and liquid sweetening processes that use caustic materials (such as caustic soda, NaOH and caustic potash, KOH). The caustic reacts with acid gases such as hydrogen sulfide, carboxylic acids, carbon dioxide, mercaptans, phenols and hydrogen cyanide. The waste product containing salts of these acids with usually some residual caustic is termed spent caustic. The spent caustic often also contains condensed and dissolved hydrocarbons and odorous organic compounds.
The traditional disposal route for spent caustic, the direct disposal of the untreated waste to the environment, is no longer acceptable and disposal to KRAFT pulp and paper mills is becoming increasingly difficult as statutory authorities worldwide move towards enforcing conservation and recycling practices at the mills.
Techniques for converting this waste to an environmentally acceptable and/or benign waste have included; oxidation followed by neutralisation and biological treatment; strong acid neutralisation followed by gas or steam stripping of the toxic acid gases and their incineration, and disposal of the neutral salt solution to the environment often after biological treatment; continuous carbonation in a combination carbonation and stripping tower; batch carbonation and stripping.
Oxidation and strong acid neutralisation processes suffer severely from the limitations imposed by the techniques available for good pH control. pH control is so variable and unreliable that it can lead to the release of toxic gases (such as hydrogen sulfide, H.sub.2 S) to the environment and low pH effluent water.
Continuous carbonation processes as described in The American Petroleum Institute Manual on Disposal of Refinery wastes, Volume III utilising common carbonation and stripping columns have not been successfully applied. This, in spite of the fact that batch carbonation and stripping processes have been successfully applied for some forty years.
The only existing successful carbonation processes are batch processes employing long residence times and small treatment volume capability.
The principal object of this invention is to provide a treatment process which improves the mass transfer of gases between phases and which can also compensate for the slowness in reaction rate inherent in many chemical reactions so as to improve (reduce) on the overall time required to complete the chemical process compared with known conventional means of carrying out the process.
A more specific object is the provision of an unique continuous carbonation and stripping process which overcomes the limitations that hinder the successful operation of existing continuous carbonation processes.